The terms "electrography" and "electrographic" as used herein broadly include various processes that involve forming and developing electrostatic charge patterns on surfaces, with or without the use of light. They include electrophotography and other processes. One method of electrographic development is the magnetic brush method which is widely used for dry development in electrophotographic document copying machines. It is disclosed, for example, in U.S. Pat. No. 3,003,462. The method of the present invention is useful in preparing the carrier particles for two-component dry developers used in the magnetic brush method. Such a developer is a mixture of thermoplastic toner particles and magnetic carrier particles, the latter of which may optionally be partially coated with an insulating resin.
In the development station of a copying machine, the two-component developer, which includes the magnetic carrier particles, is attracted to a magnetic brush consisting of stationary magnets surrounded by a rotating cylindrical sleeve or a stationary sleeve surrounding rotating magnets, e.g., as in the patent to Miskinis et al., U.S. Pat. No. 4,546,060. By frictional contact with the carrier particles, the toner particles are triboelectrically charged and cling to the carrier particles, creating bristle-like formations of developer on the magnetic brush sleeve. In developing a charge pattern, the brush is brought close to the charged surface. The oppositely charged toner particles are drawn away from the carrier particles on the magnetic brush by the more strongly charged electrostatic charge pattern, thus developing and making visible the charge pattern.
Although uncoated iron particles have been used as carriers in magnetic brush developers and although the high conductivity of uncoated iron particles is desirable because a conductive magnetic brush serves as a development electrode and improves the development of large solid areas in the image, nevertheless, resincoated carrier particles typically have been preferred. One reason for resin-coating the carrier particles has been to improve the triboelectric charging of the toner particles. When a resin-coated carrier is used, the toner powder acquires an optimally high, net electrical charge because of the frictional contact of the toner particles and the resin coating. The high net charge reduces the amount of toner lost from the developer mix as it is agitated in the magnetic brush apparatus.
Especially useful as the carrier particles in two component developers are strontium and barium ferrites. Ferrites, as used herein, are magnetic oxides containing iron as a major metallic component. The ferrites of strontium and barium referred to herein are the ferrites of strontium and barium having the formula SrO.cndot.6Fe.sub.2 O.sub.3 and BaO.cndot.6Fe.sub.2 O.sub.3. These ferrite carriers are disclosed in U.S. Pat. No. 4,546,060 to Miskinis et al and U.S. Pat. No. 4,764,445 to Saha, both of which are incorporated herein by reference. Strontium and barium ferrites, being hard magnetic materials, are desirable as carrier particles. The use of such "hard" magnetic materials which exhibit a coercivity of at least 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/g when in an applied magnetic field of 1000 Oersteds as carrier particles has been found to dramatically increase the speed of development when compared to conventional magnetic carriers made of relatively "soft" magnetic materials such as magnetite, pure iron, ferrite or a form of Fe.sub.3 O.sub.4 having magnetic coercivities of about 100 gauss or less. The terms "hard" and "soft" when referring to magnetic materials have the generally accepted meaning as indicated on page 18 of Introduction To Magnetic Materials by B. D. Cullity, published by Addison-Wesley Publishing Company, 1972.
However, a problem that has been encountered with magnetic ferrite carrier particles containing strontium and barium has been the contamination of the carrier particles with dust or fines in the form of strontium or barium oxide and/or iron (III) oxide, particularly on the surfaces of the carrier particles. When such a carrier is mixed with toner powder to form the two-component developer mixture, this dust deposits on the surfaces of the toner particles and reduces their ability to develop an electrostatic charge due to a reduction in the coercivity and induced magnetic moment caused by such contaminants. An indication of such contamination is toner "throw-off" during the development process. Throw-off is the term used to describe toner particles that separate from the carrier before they are attracted to the more strongly charged photoconductor. This phenomena may also be described as "early life dusting".
Early life dusting or toner throw-off is to be avoided for two reasons. The first reason is the potential damage such airborne toner particles or dust can do to the development apparatus in which the developer is utilized. The second reason is the imaging problems such as unwanted background development of non-image areas or portions of the element due to an incomplete discharge of such non-image areas during exposure and scumming of the electrostatic image bearing elements which are caused by such airborne toner particles. Additionally, such unattached toner particles tend to scum the carrier or pack into its pores. When this happens, the capability of the carrier for triboelectrically charging the toner particles is even further reduced. It is very important, therefore, to eliminate or significantly reduce the problem of early life dusting or toner throw-off.
The source of this contamination is a result of the way in which the ferrite carrier particles have been manufactured in the past.
In the conventional carrier manufacturing process for producing strontium and barium ferrite carrier particles, powders of ferric oxide (i.e., Fe.sub.2 O.sub.3) and the oxides of barium or strontium or a salt of barium or strontium convertible to the oxide by heat such as the carbonates, sulfates, nitrates or phosphates of barium or strontium are mixed together in a predetermined ratio, typically from about 4 to 6 moles of Fe.sub.2 O.sub.3 per 1 mole of the metal oxide or metal oxide-forming salt. This mixture of ferrite-forming precursor materials or particles is then mixed with a solution of an organic binder, such as guar gum, and a polar solvent, preferably water, ball milled into a liquid slurry and then spray dried to form unreacted, nonmagnetic, dried green beads. Spray drying is the most commonly used technique to manufacture green beads. This technique is described in K. Masters, "Spray Drying Handbook", George Godwin Limited, London, 1979, which is hereby incorporated by reference. Guar gum is a natural product which has been widely used in industry because it is inexpensive, non-toxic, soluble in water and generally available. It also undergoes nearly complete combustion in the subsequent firing stage, leaving little residue in the magnetic ferrite carrier particles. Upon evaporation, these droplets form individual green beads of substantially uniform particle size and substantially spherical shape.
During the ball milling process, a liquid slurry is produced containing the constituent raw materials. During spray drying, the solvent (e.g., water) in the liquid droplet is evaporated. In the dried droplet, the organic binder acts to bind the constituent ferrite-forming materials or particles together.
In order to prepare the magnetic carrier particles, the qreen beads are subsequently fired at high temperatures, generally ranging from about 900.degree. to 500.degree. C. During the firing process, the individual particulates within the individual green beads react to produce the desired crystallographic phase. Thus, during the firing process, the individual unreacted ferrite-forming precursor components bound in the nonmagnetic green bead react to form the magnetic carrier particles, which, like the green beads are of substantially uniform particle size and substantially spherical shape. The organic binder is degraded and is not present in the magnetic carrier particles. The magnetic character of the carrier particle, that is the coercivity and induced magnetic moment of the carrier particle is controlled primarily by the chemical stoichiometry of the constituting ferrite-forming materials and the processing conditions of reaction time and temperature. For optimum carrier performance, it is important that the chemical composition of the green beads be maintained throughout the spray drying process. The disintegration of green beads can result in chemically heterogeneous green bead particles, which will lead to less than optimum chemical reactions during the firing process and inferior magnetic performance of the final product.
It is realized, however, that this method of forming ferrites, i.e., by mechanically mixing or ball milling the constituent ferrite-forming raw materials together to a fine state of subdivision, does not achieve an intimacy in the pre-fired mixture which is conducive to rapid and complete reaction to compositionally pure strontium or barium ferrites. That is, a high degree of chemical homogenity of the precursor materials in the pre-fired mixture cannot be obtained by the mere mechanical mixing of the ferrite-forming constituent materials so that upon firing of the individual unreacted ferrite-forming precursor components bound in the non-magnetic green beads to form the magnetic carrier particles, a portion of the ferrite-forming materials do not react completely to form carrier particles of pure single-phase strontium or barium ferrite, but instead remain unreacted in the form of unwanted strontium oxide, barium oxide and/or iron (III) oxide which contaminate the carrier particles in the manner previously described herein-above.